Csf1r-Dependent Macrophages Control Postnatal Somatic Growth And Organ Maturation

Homozygous mutation of the Csf1r locus (Csf1rko) in mice, rats and humans leads to multiple postnatal developmental abnormalities. To enable analysis of the mechanisms underlying the phenotypic impacts of Csf1r mutation, we bred a rat Csf1rko allele to the inbred dark agouti (DA) genetic background and to a Csf1r-mApple reporter transgene. The Csf1rko led to almost complete loss of embryonic macrophages and ablation of most adult tissue macrophage populations. We extended previous analysis of the Csf1rko phenotype to early postnatal development to reveal impacts on musculoskeletal development and proliferation and morphogenesis in multiple organs. Expression profiling of 3-week old wild-type (WT) and Csf1rko livers identified 2760 differentially expressed genes associated with the loss of macrophages, severe hypoplasia, delayed hepatocyte maturation, disrupted lipid metabolism and the IGF1/IGF binding protein system. Older Csf1rko rats developed severe hepatic steatosis. Consistent with the developmental delay in the liver Csf1rko rats had greatly-reduced circulating IGF1. Transfer of WT bone marrow (BM) cells at weaning without conditioning repopulated resident macrophages in all organs, including microglia in the brain, and reversed the mutant phenotypes enabling long term survival and fertility. WT BM transfer restored osteoclasts, eliminated osteopetrosis, restored bone marrow cellularity and architecture and reversed granulocytosis and B cell deficiency. Csf1rko rats had an elevated circulating CSF1 concentration which was rapidly reduced to WT levels following BM transfer. However, CD43(hi) non-classical monocytes, absent in the Csf1rko, were not rescued and bone marrow progenitors remained unresponsive to CSF1. The results demonstrate that the Csf1rko phenotype is autonomous to BM-derived cells and indicate that BM contains a progenitor of tissue macrophages distinct from hematopoietic stem cells. The model provides a unique system in which to define the pathways of development of resident tissue macrophages and their local and systemic roles in growth and organ maturation.Author summary Monocytes and macrophages are cells of the innate immune system produced by the bone marrow that can be recruited into tissues to support defense against infection and repair following injury. So-called resident macrophages are abundant in every tissue in the body. Their numbers are controlled by a hormone-like growth factor called macrophage colony-stimulating factor (CSF1). Mutations in the surface receptor (CSF1R) that enables macrophages to respond to CSF1 lead to the loss of tissue macrophages. Human patients born with CSF1R mutations may die in infancy or suffer severe developmental abnormalities in the skeleton and brain. In this study we report the effects of the loss of tissue macrophages on postnatal growth and development in a rat model of CSF1R deficiency. In this model there was a global loss of resident macrophages, severe postnatal growth defects, failure of development of multiple organs including the liver and early death. The effects of the mutation could be rescued completely by transfer of normal bone marrow cells into the peritoneal cavity at weaning permitting long term survival and even fertility. The results indicate that postnatal expansion of tissue macrophage populations is essential for normal development.